Of the spray techniques used for depositing a coating on a substrate, the above technique of plasma spraying presents certain advantages, in particular the advantage of being capable of reaching high temperatures (of the order of 5000.degree. C. to 15000.degree. C.) and of obtaining specific energy densities that enable any material which has a stable molten phase to be melted. This technique can thus be applied, in particular, to ceramics that have high melting points.
Nevertheless, the plasma spray devices presently in use are limited by constraints inherent to the principle on which they operate, and which enable only certain shapes of spray to be obtained at the outlet of the nozzle and on the substrate to be coated (essentially cone-like shapes).
As a result, the spray area on the substrate cannot be polygonal in shape, and in particular it cannot be rectangular. Furthermore, in some applications and for the purpose of limiting the number of passes required for depositing a coating of given thickness, it appears to be desirable for it to be possible to widen the layer that is deposited on each pass.
Attempts have been made to mitigate these limitations by trying to obtain other spray shapes, if possible while also increasing the width of the layer deposited on each pass.
In an article entitled "A plasma nozzle with a slit-like outlet" published in 1979 in the English journal "Welding Production", Vol. 26, No. 12, pp. 32-37, a study tends to show that an increase in the diameter of the outlet orifice of the ejection nozzle gives rise to an increase in the width of the deposited layer that is very small, in fact, and on the condition that two diametrically opposite injection inlets are provided into the nozzle for the powder material. In addition, the increase in the diameter of said nozzle outlet orifice is in any event limited since it reduces the temperature of the plasma jet which may be detrimental to the melting of the particles of material injected into the plasma. In conclusion, that article proposes a compromise which consists in giving a slit shape to the outlet orifice of the ejection nozzle and in providing two diametrically opposite inlets for injecting powder material into the nozzle.
Nevertheless, according to the Applicant, such a solution is not without its own drawbacks. Going from a cylindrical shape to a conical shape between the outlet of the chamber and the inlet to the ejection nozzle causes changes in the plasma flow velocity that prevent a layer being deposited which has uniform characteristics throughout. Specifically, the Applicant considers that these drawbacks are inherent to the fact that the inert gas inserted in the chamber flows coaxially with the electric arc created between the cathode and the anode.
In general, a particular object of the invention is to improve such a plasma spray method so as to mitigate the above-mentioned drawbacks while obtaining other advantages.